3,178,274 United States Patent Office Patened Apr. 3, 1965 2 to maintain the face of the depression and the engaging 3,178,274 (OPHTHALMC GASS face of the segment free from lint, dust, finger marks and Janies E. Diacan, Natrona Heights, and Samuel L. other foreign matter. Likewise, great care must also be Seyi raoui, Galkanent, Pa., assignors to Pittsburgia Piate exercised to avoid entrapping air between the segment GiaSS CO2 agaay, Pittsbergh, Pa., a corporation of 5 face and the bottom of the depression. One general prac Peninsylvania tice is to make the face of the segment of a slightly differ No Drawing. Original application May 13, 1953, Ser. No. ent curvature than the depression so that upon heating to 734,847, agaw Patent No. 3,020,803, diated Feb. 23, fuse the major portion of the lens and segment together, 962. Divided and this applicationa May 25, 1961, Ser. the segment glass will flow into the exact curvature of the No. 12,532 depression and the entrapped air will escape during this 3 (Clains. (C. 65-39) 0. heating. Another general practice is to assemble the ma This invention relates to an ophthalmic glass and it has jor portion of the lens and the segment with one side of particular relation to a glass having a relatively high the segment slightly elevated by the introduction of a Softening point and a high index of refraction so as to be small glass or wire wedge between the segment and the useful as the segment in a particular method of forming 5 major portion of the lens. The assembly is heated to a a multifocal, ophthalmic lens. temperature such that the segment glass softens, settles A multifocal lens is conventionally composed of two and fuses within the depression and in so doing, pushes different ophthalmic glasses. A major portion of the lens the air out of the interspace between the elements, the is composed of a crown glass or other glass which has a fusing taking place gradually from one side of the depres relatively low index of refraction and which is highly re 20 sion to the other. All of these methods involve the use fined and free from seeds, striae and other imperfections of the conventional crown and segment glasses mentioned which would impair the optical properties of the lens. above. The crown glass has a ground and polished recess in it A novel method of forming a multifocal lens blank is and a glass in the form of a small segment is fused in the disclosed in an application of Samuel L. Seymour, Serial recess. The segment glass has a higher index of refrac 25 No. 688,560, filed October 7, 1957. In this process, a tion than the crown glass and it is the portion of the fused mold is used which conforms to the shape of the multi multifocal lens which is used for reading. focal lens blank and which has a recess or countersink of One example of a crown glass is an alkali-lime silica configuration which is complementary to the segment. glass containing approximately 70 percent SiO2, 8.5 per The segment, having an optically finished surface on one cent Na2O, 7.5 percent K2O, 12 percent CaO and 2 per 30 side thereof, is preheated to a temperature below its soft cent refining agents such as Sb2O3. The segment is ening point and placed within the recess or countersink usually made from highly refined glasses known as with its finished surface facing upwardly. Molten glass barium, flint, barium-flint or barium crown glasses. Ex of optical quality to form the major portion of the multi amples of these glasses are shown in U.S. Patents Nos. focal lens blank is then fed from a glass furnace discharge 2,523,264, 2,523,265, 2,523,266 and 2,528,634. The orifice onto the mold surface in such a manner that the crown glass and the segment glass preferably have sub molten glass does not contact the preformed segment but stantially the same coefficient of thermal expansion. The has a part of its perimeter in close proximity thereto. A barium type glasses usually have a slightly lower soften molten mass of glass is allowed to accumulate in the mold ing point than the crown glasses and the flint glasses and the feed is stopped. Immediately thereafter, the usually have a softening point considerably lower than the 40 accumulated mass of molten glass is laterally pushed so crown glasses. as to flow over the mold surface and the finished surface In the manufacture of a multifocal lens, a processed, of the heated segment. In so doing, the molten glass segment blank is fused to a processed major blank. The fuses to the segment without the entrapment of air at the major blank and the segment blank are made in a great interface or area of fusion. Subsequently, if necessary variety of shapes and sizes by the glass manufacturers. or desirable, the distributed mass, while still in its molten These blanks are semi-finished by the various lens proc 45 condition, may be subjected to a vertically directed press essing companies and are usually assembled in the manner ing with a suitably shaped molding member to further hereinafter described. The face of the segment blank to complete the shaping of the composite multifocal lens be engaged by the major blank is ground and polished to blank. stock minor focal fields usually expressed in diopters. This novel method of manufacture of a multifocal lens A suitable polished depression or countersink is formed in 50 blank requires the use of a segment glass having a soften the major blank to receive the segment. The segment is ing point which is much higher than that of the high in placed in the depression and the assembly is heated to dex, segment glasses which are used in the conventional fuse the segment glass to the glass used for the major processes described above. The segment glass which is blank. Thereafter, the fused bifocal biank is ground and employed in the novel method should have a softening polished on both sides to form a semi-finished lens. 55 point which is higher, for example, 100 to 150 F. or The perimeter of the segment blank is not always of more than the softening point of the glass which forms circular form. Frequently, it is in the form of a semi the major portion of the multifocal lens in order to avoid circle, rectangle or modification thereof. The various distortion of the finished curved surface of the segment shapes make it difficult to form correspondingly shaped during fusing of the molten major portion glass to the segment receiving depressions in the major blank. One 60 segment. usual practice is to grind and polish in the major blank The present invention is concerned with providing a a circular depression which is large enough to receive the glass for use as the segment glass in the novel method of Segment. Any remaining portion of the depression sur manufacture of multifocal lens blanks as described above, rounding the segment is filled with smaller shaped por which glass has a relatively high softening point, i.e., from tions of crown glass which are fused to the segment prior 65 1385 F. to 1475 F., an index of refraction, N, between to fusion of the segment in the depression. The whole 1.57 and 1.67, a high reciprocal dispersion ranging from assembly is then fused together so that only the segment 40 to 56 and a coefficient of expansion which is compatible is visible apart from the major portion of the lens when with the glass of the major portion of the lens within the the lens is ground and polished. range of 7 to 10 x 106 per C. between 25 C. and 300° C. When assembling the segment in the depression of the 70 Novel glasses within the purview of the invention are pre major portion of the lens, great care must be exercised pared by the use of conventional glass making materials in 3,178,274 3 4. conventional, ophthalmic glass manufacturing equipment. the temperature of the furnace is gradually lowered in The following batches in pounds and compositions in cal about 34 of an hour to about 2075 F. A furnace tem culated percent by weight produced therefrom are exem perature of about 2075 F. is then maintained for a period plary of the glasses of the invention: of about 4 hour. The pot of glass is then removed from BATCHES

Ingredients Pounds

Sand------Barium Carbonate Sodium Carbonate Potassium Carbonate Calcium Carbonate Zinc Oxide------

Sodium Nitrate---- Antimony Trioxide Arsenic Trioxide--- Aluminum Hydrate Calcium Fluoride-- Sodium Silico Fluoride--

Component range 1

4.4. : 3 : 2 9. 4. 3. 3. 3. 2. 7. i.

0-5. Index of Refraction Nd------1.618 1.590 1620 Coeff. of expansion X 10.6 per C. 1, 57-167 between 25 C. and 300 C------9.0 Softening point F. (temp. at which 8.5, 8.9 7...0-0. the log of the vis, in poises is 1076--- 1,389 Reciprocall dispersion (Nd-1)/ 1,385 1,430 1,406 1,396 1,403 1,385-1,475 NFNc------49.8 50.7 50.2 54.6 49.0 42.4 43.2 40-56 The softening point recited in the table is defined as the the furnace, the glass is poured on a metal table and is temperature in degrees Fahrenheit at which the viscosity rolled into the form of a sheet. The sheet is placed in a of the glass is 107.6 poises. This viscosity is obtained ac kiln and cooled from 1050 F. to 850 F. at a rate of about cording to the method described on pages 228 to 231 of the 5 F. per minute. Thereafter, it is cooled more rapidly 1957 supplement to the Book of ASTM Standards, a pub to room temperature and cut into pieces suitable for lication of the American Society for Testing Materials, preliminary property tests. copyright 1958. - This glass may be further refined to improve its optical In the preparation of the glasses, the batch ingredients 50 quality. The glass is broken into cullet and about 15 are thoroughly mixed in the proportions necessary to pro pounds is placed in a platinum lined clay crucible which duce the glasses. Various size pots or crucibles may be has been preheated to a temperature of about 2000 F. employed and the temperatures and times will vary ac The crucible and contents are placed in an electrically cording to the amount of glass being formed. The tem heated furnace which is at a temperature of about 2200 peratures and melting conditions herein recited are em 55 F. The furnace temperature is gradually raised at a uni ployed to make 85 to 100 pounds of these glasses in clay form rate to about 2670 F. over a period of about 2% pots in a furnace heated by the controlled combustion of hours. This temperature is maintained for a period of natural gas. about 1/2 hours during which time the bubbles are sub The empty pot is preheated in the furnace at a furnace stantially eliminated from the melt. temperature of about 2200 F. A portion of the mixed 60 At the end of this period, a platinum stirrer is inserted batch is ladled into the preheated pot and the furnace in the molten glass. The glass is stirred while the tem temperature is gradually increased. The remaining por perature of the furnace is gradually reduced over a pe tion of the mixed batch is ladled into the pot at a uniform riod of 94 hour to a temperature of 2075 F. A furnace rate over a period of 3% hours and the temperature is temperature of 2075 F. is maintained for a period of A raised gradually during this period to about 2600 F. 65 hour while stirring is continued. The stirrer is then re During the next /2 hour the furnace temperature is fur moved from the glass and the crucible is removed from ther increased to between 2650 and 2670 F. and within the furnace. The glass is poured on a metal table and this time substantially all of the glass making materials rolled into the form of a sheet. The sheet is placed in a are melted. This temperature is maintained for the foll kiln and cooled from 1050 to 850 F. at a rate of about lowing 1/2 hours during which time the chemical reac 70 5 F. per minute. Thereafter it is cooled more rapidly tions are completed and the glass becomes substantially to room temperature and cut into pieces suitable for free of bubbles. During the melting and high temperature processing according to conventional techniques into seg reacting periods just described a neutral or slightly oxidiz ments. ing atmosphere is maintained within the furnace. In the manufacture of the glasses of the invention, silica After the glass has become substantially free of bubbles, 75 is the principal glass former. If lower than 35 percent 3,178,274 5 6 by weight of SiO2 is employed in the composition, it tends K2O, 18.6 percent PbO, 0.5 percent TiO, 0.5 percent to reduce the chemical durability of the glass, whereas Al2O3 and 0.8 percent Sb2O5. These crown glasses have amounts higher than 52 percent by weight of SiO, make a coefficient of expansion between about 7 to 10x10-6 it increasingly difficult to achieve a glass having a high per C. between 25 and 300° C., an index of refraction index of refraction. between 1.520 and 1.540 and a softening point of about The total of the alkali metal oxides, NaO and KO, 1125 to 1250 F. The softening point of the segment is maintained between 6 and 12 percent by weight. A glass should be at least 100 F. or more above the soften minimum amount of alkali metal oxide is required to ing point of the crown glass and it is preferred that the achieve the desired coefficient of expansion, but an excess softening point of the segment glass be from 135 to 350 is to be avoided for it has a deleterious effect on the chem 0 F. or more above the softening point of the crown glass ical durability of the glass and it also lowers the softening when utilized in the novel process. point. It is desired that no lithia be present in the glasses In the manufacture of the multifocal lens blanks ac of the invention, for lithia greatly lowers the softening cording to the novel process, the blank mold is heated point of the glasses. to a temperature of about 700 to 800 F. A segment A relatively large amount of barium oxide is desired glass having a composition as set forth in Glass No. 1 to achieve the high index of refraction, but too much of the table and having an optically finished surface is barium oxide causes the glasses to devitrify. Careful se preheated to a temperature of about 1250 to 1300 F. lection and control of the other ingredients of the glasses and placed within a recess or countersink within the permits the use of the high amount of barium oxide to heated mold with its finished surface facing upwardly. obtain the unusually high softening point and prevent de 20 Molten crown glass of optical quality made according to vitrification of the glasses of the invention. Calcium ox conventional continuous optical glass manufacturing ide, zinc oxide and lead oxide are present to provide the techniques is simultaneously deposited onto the mold sur required properties and prevent devitrification. Cadmium face in such a manner that the molten glass does not oxide and strontium oxide may also be used for this pur contact the preformed segment, but has a part of its pose. Any one or combination of the oxides of calcium, perimeter in close proximity thereto. This glass may zinc, lead, cadmium and strontiuin in an amount between have a composition the same as that of the exemplary 5 to 15 percent by weight is suitable. glass set forth in the preceding paragraph. A molten Zirconium dioxide is necessary to produce a glass hav mass of glass is allowed to accumulate in the mold and ing a high softening point; however, an excess of zirconium the feed is then stopped. The temperature of the molten dioxide undesirably reduces the coefficient of expansion 30 glass is about 1875 to 1975 F. Immediately there of the glass. About 0.5 to 5 percent by weight of titanium after the accumulated mass of molten glass is laterally dioxide may be used to control the refractive index of the pushed by a molding member heated to a temperature glass. Its use is restricted to less than about 5 percent of about 700 to 800 F. so as to flow the glass over because it lowers the reciprocal dispersion unduly. No the mold surface and the finished surface of the minor boron oxide is present in the glasses of the invention. It 35 segment and to fuse to the segment without the entrap has been found that a combination of BO and the rela ment of air at the interface or area of fusion. The tively high amount of BaO present in the glasses of the molding member is then vertically impressed on the invention adversely affects their chemical durability. molten glass to further shape the multifocal lens blank. Approximately 95 or more percent by weight of the The blank is removed from the mold and cooled slowly glasses which are the subject of this invention is comprised 40 in an annealing lehr from a temperature of 1050 to of SiO2, BaO, ZrO2, Na2O, K2O and one or more bivalent 850 F. at a rate of 5 F. per minute. The multifocal metal oxides selected from the group consisting of CaO, lens blank may then be ground and polished accord ZnO, PbO, CdC) and SrO. The remaining approximately ing to conventional ophthalmic procedures to a finished 5 percent or less by weight of the glasses may be made lens suitable for incorporation in spectacles. up of TiO2, fining agents, melting aids and other materials 45 Although the present invention has been described such as colorants which may affect the transmission, ab with reference to specific details of certain embodiments sorption or other properties of the glasses without unduly thereof, it is not intended that such details shall be lowering their softening point or harmfully affecting their regarded as limitations upon the scope of the inven other desirable optical properties. tion except insofar as set forth in the following claims. For example, fluorine may be present to act as an aid in 50 This application is a continuation-in-part of our co refining the glass at high temperatures. Its use is restrict pending application Serial No. 478,518, filed December ed to less than about 1.5 percent by weight because when 29, 1954, now abandoned, and is a division of our co used in larger amounts objectionable opalescence forms pending application Serial No. 734,847, filed May 13, in the glass due to precipitation of fluoride crystals in the 1958, now U.S. Patent 3,020,803. body of the glass. Aluminum oxide may also be included 55 We claim: in the glasses to help obtain the high softening point. Its 1. In the method of forming a multifocal lens blank use is limited to less than about 5 percent by weight be which comprises placing a segment having an optically cause higher amounts have a tendency to cause the glasses finished surface in a mold with the optically finished to devitrify. The oxides of antimony and arsenic are Surface facing upwardly, depositing a molten mass of a added as conventional fining agents and other oxides or 60 crown glass onto the mold surface in such a manner other compounds which act by themselves or collectively that the molten glass does not contact the segment glass as fining agents may be employed in the practice of the and laterally pushing the molten mass of crow glass over invention. the mold surface and the finished surface of the seg The glasses of the invention may be utilized as segments ment to fuse the molten glass to the segment glass with in the novel process described in the previously mentioned 65 out the entrapment of air at the interface or area of application of Samuel L. Seymour in combination with fusion, the softening point of the segment glass being conventional crown glasses and preferably with special low at least 100 F. higher than the softening point of the softening point crown glasses containing 62 to 66 percent crown glass, the improvement which comprises using SiO2, 0 to 15 percent NaO, 0 to 15 percent KO, the as the segment glass a boron-free and lithia-free glass sum total of alkali metal oxides being 12 to 17 percent, 70 having an index of refraction, Nd, between 1.57 and 15.2 to 20 percent PbO, 0.5 to 4 percent TiO, and 0.5 to 1.67, a softening point above 1385 F. and a coefficient 3 percent Al2O3 as disclosed in another copending appli of thermal expansion of 7 to 10x10-6 per C. be cation of Samuel L. Seymour, Serial No. 538,516, filed tween 25 and 300° C. which consist essentially of the October 4, 1955. The composition of an example of such following ingredients in percent by weight: 35 to 52 per a glass is 65.0 percent SiO2, 5.3 percent Na2O, 9.3 percent cent SiO2, 0 to 12 percent Na2O, 0 to 10 percent KO, 3,178,274 7 3 the sum of NaO and KO being 6 to 12 percent, 28.1 0 to 12 percent Na2O, 0 to 10 percent KO, the sum to 34 percent BaO, 2 to 15 percent ZrO2, 0 to 10 percent of Na2O and KO being 6 to 12 percent, 28.1 to 34 per CaO, 0 to 10 percent ZnO and 0 to 10 percent PbO, cent BaO, 2 to 15 percent ZrO2, 0.5 to 5 percent TiO, the sum of CaO, ZnO and PbO being 5 to 15 percent, 0 to 10 percent CaO, 0 to 10 percent ZnO and 0 to 10 the above listed essential ingredients constituting at least percent PbO, the sum of CaO, ZnO and PbO being 5 95 percent by weight of the glass. to 15 percent. 2. A boron-free and lithia-free glass having an index of refraction, Nd, between 1.57 and 1.67, a softening References Cited by the Examiner point of at least 1385 F. and a coefficient of thermal UNITED STATES PATENTS expansion of 7 to 10x10-6 per C. between 25 and O 300° C. which consists essentially of the following in 2,391,468 12/45 Long ------117-129 gredients in percent by weight: 35 to 52 percent SiO2, 2,433,013 12/47 Ziegler ------49-82.1 0 to 12 percent Na2O, 0 to 10 percent K2O, the sum of 2,491,965 12/49 Ganci ------106-48 X NaO and KO being 6 to 12 percent, 28.1 to 34 percent 2,523,264 9/50 Armistead ------106-53 Bao, 2 to 15 percent ZrO2, 0, to 10 percent CaO, 0 to 5 2,523,265 9/50 Armistead ------106-53 10 percent ZnO and 0 to 10 percent PbO, the sum of 2,660,531 11/53 Fraser ------106-48 CaO, ZnO and PbO being 5 to 15 percent. 2,699,399 1/55 Armistead ------106-53 3. A boron-free and lithia-free glass having an index of refraction, Nd, between 1.57 and 1.67, a softening FOREIGN PATENTS point of at least 1385 F. and a coefficient of thermal 20 837,727 6/60 Great Britain. expansion of 7 to 10x10-6 per C. between 25 and 300° C. which consists essentially of the following in TOBLAS E. LEVOW, Primary Examiner. gredients in percent by weight: 35 to 52 percent SiO2, JOSEPH REBOLD, Examiner.